Chemical transformation of zinc passivating products after carbonation process

The composition of passivating products of zinc in Ca(OH)2 saturated solution after carbonation process was studied. In a previous research, carried out on galvanized steel sheets embedded in concrete submitted to carbonation, the layer of calcium hydroxyzincate Ca[Zn(OH)3]2 ⋅2H2O (CHZ), which determines the passivity state of zinc in concrete, was found to be destroyed by carbonation process. However, it wasn’t possible to identify the products resulting from CHZ transformation, because they are no well crystallized. In the present work, Ca(OH)2 saturated solution was used, in place of concrete matrix, to accelerate CHZ reaction with CO2 in order to obtain a greater quantity of the formed compounds to be analyzed by X-ray diffractometry and Raman spectroscopy. The results obtained indicate that the products of the reaction of CHZ with CO2 are Zn5(CO3)2(OH)6 (hydrozincite) and CaCO3 (calcite). Corrosion potential and corrosion rate measurements show that, despite the deep change in the passivating products, zinc maintains its passive state also after carbonation, according to previous results obtained in concrete and according to bibliography.